1. Field of the Invention
This invention relates to oxygen enrichment in industrial heating applications, in particular in power boilers.
2. Related Art
In a previous disclosure of the same assignee of the present invention, Air Liquide Serie file 5167, filed Nov. 10, 1999, application Ser. No. 09/437,526, now U.S. Pat. No. 6,314,896, issued Nov. 13, 2001, there was a proposed scheme of oxygen-enrichment in boilers using large amounts of oxygen, up to full oxy-fuel combustion. That patent application involved a certain ration between the oxygen enrichment and the flue gas recirculation, such that the design boiler parameters are maintained constant.
While quite inventive, the above-referenced methods in said patent application may not be desirous in particular industrial heating applications, in particular coal fired boilers, primarily pulverized coal, but with the application to fluidized beds also. Coal combustion results in a potentially large amount of unburned coal in the stack, thus losing a large amount of fuel. Also, due to the incomplete combustion of coal, as well as to the sometimes difficult ignition process, a support fuel such as natural gas is frequently used in significant quantities (from about 10% to about 50% of the total amount of fuel). The ease and completeness of combustion is directly related to the volatile content of the coal, and indirectly related to the percentage of moisture in the coal. In other words, more moisture means more difficult and possibly incomplete combustion, while more volatiles in the coal means more complete combustion.
FIG. 1 presents in schematic, a prior art combustion chamber 2 in a boiler 4, where the combustion chamber 2 is divided into two major zones: Zone I, as denoted in FIG. 1 at 6, represents the area where combustion burners are located, together with air inlets. Combustion air can enter combustion chamber 2 together with a fuel (part of the air is used to transport coal into combustion chamber 2), or in different inlets. Combustion air can be introduced into the boiler partially or totally at this location. More modern schemes use a different air inlet, denoted as Zone II, and noted as 8 in FIG. 1, in order to improve the combustion process and to lower the NOx emissions. The combustion scheme illustrated schematically in FIG. 1 is termed xe2x80x9cstaged combustionxe2x80x9d since the combustion process occurs in two zones. It is noted that the schematic in FIG. 1 is very general, showing a generally horizontal flue gas circulation 10. In general, flue gas circulation can be in any direction (vertical, horizontal, circular, and the like).
As illustrated in FIG. 1, the combustion process is divided into two major zones: Zone I represents the ignition zone, where the fuel(s) enter the combustion chamber, are heated, and ignite. When coal is of a lesser quality, additional fuel (generally natural gas or fuel oil) is required for a fast ignition. Zone II represents the final region allocated for combustion. Additional oxidant may be introduced, as mentioned supra. Modern staged combustion schemes allow a significant portion of the oxidant to enter Zone II (between 10% and 50% of the total oxidant). Due to the low pressure of the incoming air in Zone II, the flow patterns of the main flue gas stream are relatively undisturbed, thus the mixing between the two streams is relatively poor, preventing the full combustion of the fuel. This is represented by the shaded area 16 in FIG. 2. FIG. 2 illustrates that the mixing zone 16 between the flue gas stream 10 and the balance of the oxidizer in Zone II and exiting into the mixing zone 16 is not total, thus an important part of the fuel will not mix with the oxidant, thus remaining unburned.
It would be an advantage if the fuel from Zone I could be mixed with oxidant from Zone II to provide better mixing between fuel and secondary oxidant.
In accordance with the present invention, methods and apparatus are provided which overcome problems associated with the prior art methods. The present invention involves introducing a high velocity stream of an oxygen-enriched gas into Zone II through a multitude of streams, preferably uniformly distributed for better mixing. xe2x80x9cOxygen-enrichedxe2x80x9d as used herein, is considered any gas having concentration of oxygen higher than 21% (the oxygen concentration in air). The results of this inventive process are: providing the fuel and/or fuel-rich combustion products with enhanced oxidant (when compared to air), and also improving the mixing between the fuel and/or fuel-rich combustion products and the oxidant. The combined effect of high oxygen concentration and improved mixing leads to a more effective and complete fuel combustion.
One aspect of the invention is a method of increasing combustion of a hydrocarbon fuel in a combustion chamber of a furnace, the combustion normally using only air as an oxidant, part of the air entering the combustion chamber near (preferably in) one or more fuel burners, and a remaining portion of air entering the combustion chamber at a plurality of locations downstream of said fuel burners, the method comprising the steps of injecting oxygen-enriched gas through a plurality of lances into a flue gas in the combustion chamber at the plurality of downstream locations, the oxygen-enriched gas injected at a velocity ranging from subsonic to supersonic, the oxygen-enriched gas being present in an amount sufficient to provide an oxygen concentration in the flue gas of no more than 2% on a volume basis greater than when air is used alone as oxidant. Preferably, the velocity is subsonic for the oxygen-enriched gas in each of the plurality of lances, or in some embodiments the velocity is supersonic for the oxygen-enriched gas in each of the plurality of lances. In any case the velocity of the oxygen-enriched gas is greater than velocity of air injection.
As used herein the term xe2x80x9ccombustion chamberxe2x80x9d includes any area where combustion of fuel can occur in a furnace or boiler.
In other preferred embodiments, some of the oxygen-enriched gas is injected at subsonic velocity in one or more lances while a balance of the oxygen-enriched gas is injected at supersonic velocity through one or more lances.
The oxygen-enriched gas is preferably injected through the lances at an angle with respect to a wall of the combustion chamber, the angle ranging from about 20xc2x0 to about 160xc2x0, the angle measured in a plane that is perpendicular to the wall. Preferably, the plurality of locations are arranged so that one-half of the lances are on a first wall and one-half of the lances are on a second wall. Also preferred are embodiments where lances on the first wall are separated by distance LL, wherein LL less than LCH/2, wherein LCH is selected from the group consisting of height, length, and width of the combustion chamber, and wherein the lances on the first wall are positioned a distance 1 from the lances on the second wall, wherein 0 less than 1 less than LL/2, wherein LL is the distance between lances on the first wall.
Preferably, the combustion chamber is rectangular, wherein there is one lance on each of four walls of the rectangular combustion chamber, and wherein each lance is a distance Lxe2x80x2 from a wall wherein an adjacent lance is positioned. Preferably, Lxe2x80x2 less than LCH/2. In this embodiment, each lance is preferably positioned at a first angle ranging from about 20xc2x0 to about 160xc2x0, the first angle measured in a first plane which is substantially vertical and substantially perpendicular to its corresponding wall, and each lance is preferably positioned at a second angle ranging from about 20xc2x0 to about 160xc2x0, the second angle measured in a plane substantially perpendicular to the first plane.
In the embodiments which employ rectangular combustion chambers, the remaining portion of air preferably enters the combustion chamber through one or more rectangular slots, or through one or more substantially circular slots.
Other preferred methods are those wherein the oxygen-enriched gas is injected in substitution for the remaining portion of air, and methods wherein said oxygen-enriched gas is injected into the remaining portion of air.
A second aspect of the invention is a method of increasing combustion of coal in a combustion chamber of a furnace, the combustion normally using only air as an oxidant, part of the air entering the combustion chamber near (preferably in) one or more fuel burners, and a remaining portion of air entering the combustion chamber at a plurality of locations downstream of the fuel burners, the method comprising the steps of injecting oxygen-enriched gas through a plurality of lances into a flue gas in the combustion chamber at the plurality of downstream locations, the oxygen-enriched gas injected at a velocity ranging from subsonic to supersonic, and the oxygen-enriched gas being present in an amount sufficient to provide an oxygen concentration in the flue gas of no more than 2% on a volume basis greater than when air is used alone as oxidant. As in the first aspect, the injected oxygen-enriched gas is injected at a velocity greater than the air would have been.
Preferred are those methods wherein some of the oxygen-enriched gas is injected at subsonic velocity in one or more lances while a balance of the oxygen-enriched gas is injected at supersonic velocity through one or more lances.
Also preferred are methods within this aspect wherein the oxygen-enriched gas is injected through the lances at an angle with respect to a wall of the combustion chamber, the angle ranging from about 20xc2x0 to about 160xc2x0, the angle measured in a plane that is perpendicular to the wall.
Preferred embodiments with the second aspect include those methods wherein the plurality of locations are arranged so that one-half of the lances are on a first wall and one-half of the lances are on a second wall; methods wherein lances on the first wall are separated by distance L, wherein L less than LCH/2, wherein LCH is selected from the group consisting of height, length, and width of the combustion chamber; methods wherein the lances on the first wall are positioned a distance 1 from the lances on the second wall, wherein 0 less than 1 less than L/2, wherein L is the distance between lances on the first wall; methods wherein the combustion chamber is rectangular, and wherein there is one lance on each of four walls of said rectangular combustion chamber, wherein each lance is a distance Lxe2x80x2 from a wall wherein an adjacent lance is positioned; methods wherein each lance is positioned at a first angle ranging from about 20xc2x0 to about 160xc2x0, the first angle measured in a first plane which is substantially vertical and substantially perpendicular to its corresponding wall; and methods wherein each lance is positioned at a second angle ranging from about 20xc2x0 to about 160xc2x0, the second angle measured in a plane substantially perpendicular to the first plane.
Preferably, the remaining portion of air enters the combustion chamber through one or more rectangular slots, or through one or more substantially circular slots.
Preferably, the oxygen-enriched gas is injected in substitution for the remaining portion of air, or the oxygen-enriched gas is injected into the remaining portion of air.
A third aspect of the invention is a method of increasing combustion of a hydrocarbon in a combustion chamber of a furnace, the combustion normally using only air as an oxidant, part of the air entering the combustion chamber near (preferably in) one or more fuel burners in a first zone of the combustion chamber, and a remaining portion of air normally entering the combustion chamber at a plurality of downstream locations, the method comprising injecting a first portion of oxygen-enriched gas into the combustion chamber at the plurality of downstream locations, the first portion of oxygen-enriched gas injected at a velocity ranging from subsonic to supersonic, wherein the first portion of oxygen-enriched gas is injected through a centrally located oxygen lance, which injects the oxygen-enriched gas into a flame created at each of the plurality of locations by a second portion of oxygen-enriched gas and a fuel. Preferably, the totality of oxygen-enriched gas is injected at an amount sufficient to provide an oxygen concentration of no more than 2% greater than when air is used alone.
As with the previous aspects of the invention, the first portion of oxygen-enhanced gas may be either injected at sub-sonic or supersonic velocity, but in all cases, greater velocity than if air were used alone.
Preferred are methods wherein said second fuel is selected from the group consisting of gaseous and liquid fuels, and wherein the second portion of the oxygen-enriched gas has substantially the same concentration of oxygen as the first portion of oxygen-enriched gas; also preferred is when each lance is positioned at a first angle ranging from about 20xc2x0 to about 160xc2x0, the first angle measured in a first plane which is substantially vertical and substantially perpendicular to its corresponding wall.